The invention is in the field of electronic reproduction technology and is directed to a method for generating and evaluating a sample engraving in an electronic engraving machine for engraving printing cylinders for rotogravure.
In an electronic engraving machine, an engraving element with an engraving stylus as a cutting tool moves in an axial direction along a rotating printing cylinder. The engraving stylus controlled by an engraving control signal cuts a sequence of cups arranged in an engraving screen into the generated surface of the printing cylinder. The engraving control signal is formed by superimposition of image signal values, which represent the tone values to be engraved between “light” white and “dark” black, with a periodic screen signal. Whereas the screen signal effects a vibrating lifting motion of the engraving stylus for generating the engraving screen, the image signal values determine the geometry values of the cups engraved into the printing cylinder.
The engraving control signal must be calibrated so that the engraved tone values correspond to the hues defined by the image signal values. For that purpose, what is referred to as a sample engraving is implemented before the actual engraving, sample cups for predetermined tone values being engraved into the printing cylinder in this sample engraving.
After the sample engraving, a measuring instrument is positioned on the engraved sample cups and their geometry values such as, for example, the transverse diagonals and longitudinal diagonals are measured.
The measured geometry values of the sample cups are then compared to the predetermined geometry values. Setting values are obtained from the comparison with which the engraving control signal is calibrated such that the geometry values of the cups generated in the later engraving coincide with the geometry values required for reproduction with proper tone values.
The PCT patent application serial number PCT/DE 98/01441 has already disclosed that a video camera with an image evaluation unit be employed for determining the geometry values of engraved sample cups, the geometry values in a video image of the sample cups registered with the video camera being measured with said image evaluation unit.
A prerequisite for an exact measurement is that the sample cups fall completely into the image excerpt registered by the video camera after a manual or automatic positioning of the video camera given optimum image resolution. This condition is not always met in practice, particularly after changing engraving styli, and mismeasurements are the result.